Ruthenium (Ru) is expected to be introduced in the industrial semiconductor manufacturing process for many applications in the coming years. This move towards the use of new materials for chip manufacturing is necessary to solve issues generated by the continuous scaling trend imposed to the industry. For the next generation nodes, Ru is considered as the best candidate for the electrode capacitor for FeRAM and DRAM applications. Ru has the required properties, such as high melting point, low resistivity, high oxidation resistance and adequate work function, making it a potential gate electrode material for CMOS transistor. Ru has advantages compared to iridium and platinum due to its lower resistivity and Ru ease of dry etching. Additionally, RuO2 has a high conductivity so the formation of Ru oxide by diffusion of oxygen, that could come from ferroelectric films (PZT, SBT, BLT, . . . ), will have less impact on electrical properties than other metal oxides known to be more insulating.
Ru is also a promising BEOL process candidate as a liner material for copper. The poor wettability of Cu on currently used Ta/TaN barrier stack layers does not allow to directly deposit copper on such materials. The use of ruthenium between Ta/TaN and Cu is necessary to allow high quality thin copper gas-phase deposition with required adhesion strength to the Ru film prior to the ECD filling.
A large variety of Ru compounds are available and many have been studied as CVD precursors. However, the deposition process developed with these chemistries currently does not provide an answer the challenges of down-scaling imposed by the industry. Among other reasons, the need for an oxidant source to react with the ruthenium precursor (such as Ru(EtCp)2, Ru(EtCp)(Op), Ru(Me2-pyrollyl)2) is considered as a source of damage to the under-laying nitride film, as the surface of such film will oxidize, generating less conductive interface. This will have a negative impact on the RC delay. Long incubation times are also frequently reported with the above mentioned precursors. This obviously generates control and production issues at the industrial stage (see T. Shibutami et al, Tosoh R&D Review, 47, 2003; K. Kukli et al., Thin Solid Film, 520 (2012) 2756-2763).
In order to avoid the use of an oxidant source, it is considered that ruthenium compounds having an oxidation state 0 should be used. With such compounds, pure ruthenium films can be used without co-reactant (see US20070072415, WO2008034468). A saturation regime typical of an Atomic Layer Deposition (ALD) mode may not be secured, but this is of relatively low concern given the relatively low aspect ratio patterns faced in BEOL application. However, a problem currently faced with such oxidation state 0 molecules is that they contain a carbonyl ligand, CO, which may damage the under-laying films.
Aside from the above mentioned precursors, some diazabutadiene based molecules have been developed. Diazabutadiene (DAD) ligands are α-diimine ligands that may be used under different oxidation states. The DAD ligand may be selected from one of three oxidation state forms, with each form determining the bonding mode between the center element (M) and the DAD ligands. As used herein, three different oxidation states of the ligand are described as i) neutral, ii) mono-anionic, and iii) dianionic. One of ordinary skill in the art will recognize that the location of the double bonds in the diazabutadiene ligand changes based upon the oxidation state of the ligand, as shown below:

In Organometallics 1986, 5, 1449-1457, H.T. Dieck prepared the arene diazadiene ruthenium complex with the oxidation state 0. The complexes are prepared in two steps by preparing first the arene diazadiene chloro ruthenium (II) tetrafluoroborate complex followed by a reduction step with sodium naphtalene. When the complex is Ru(benzene) (dimethylglyoxal(bis-isopropylimine)) the final material has a melting point slightly above room temperature and can be purified by sublimation at 80° C. under vacuum.
A need remains for ruthenium containing precursors suitable for CVD or ALD using a ruthenium molecule that would have an oxidation state 0, and would not contain oxygen in its structure, and that such molecule allows the deposition of pure ruthenium films without any oxidant source with high enough uniformity onto the considered structures.